This invention concerns an apparatus for measuring the flow rate of a fluid such as a gas or liquid.
Various kinds of devices for measuring the flow rate of a gas or liquid are known.
The first example is an orifice type flow meter adapted to measure a differential pressure (.DELTA.p) generated between both sides of an orifice disposed within a flow channel, by means of a differential pressure detector. According to this flow meter, the volumetric flow rate (Q) of a fluid flowing through the flow channel is determined as: ##EQU1##
The second example is a pressure receiving plate type flow meter adapted to measure the force (Fn) of a fluid acting on a pressure receiving plate disposed within a flow channel. In this flow meter, the volumetric flow rate (Q) of a fluid flowing in the flow-channel is determined as: ##EQU2## where
.rho.: density of the fluid
Fn: force acting on the pressure receiving plate.
In the orifice type flow meter of the first example as described above, detection of a small flow rate Q is difficult since only a slight differential pressure .DELTA.p is generated for small flow rates. On the other hand, if the flow rate Q is greater, since the differential pressure .DELTA.p is increased in proportion to the square of the flow velocity V, the differential pressure detector has to detect the differential pressure over a wide range. However, the detector capable of satisfying such a requirement is often expensive. Further, since the cross-sectional area in the flow channel of the orifice is constant, the pressure loss in the orifice is increased as the flow rate Q increases.
In order to overcome the foregoing drawbacks, there are known variable orifice type flow meters in which the bellows (or equivalent) of the flow meter are made movable responsive to the force of a fluid, thereby varying the cross-sectional area of the orifice flow channel. However, this flow meter is complicated in structure and is not suitable to the measurement of a small flow rate.
The pressure receiving plate type flow meter mentioned above as the second example is also not suitable to the measurement of a small flow rate since the volumetric flow rate is proportional to the square root of the force (Fn) acting on the pressure receiving plate.